Coating compositions have long been used to provide the surface of articles with certain desired physical characteristics, such as color, gloss and durability. Many coating compositions rely on a liquid carrier, which evaporates after the composition is applied. In recent years, powder coatings have become increasingly popular; because these coatings are inherently low in volatile organic content (VOCs), their use reduces air emissions during the application and curing processes as compared with liquid coatings.
Powder coatings are typically cured by heating the coated substrate to an elevated temperature. These temperatures almost always exceed 125° C., and commonly reach about 190° C. to 205° C. During the curing process, the powder particles melt, flow, and coalesce, and the components of the powder coating react. In addition to not emitting any VOCs into the environment during the application or curing processes, powder coating systems are extremely efficient since there is essentially no waste (i.e., application yield is approximately 100 percent). Because of the relatively high (i.e., greater than 125° C.) cure temperatures of most powder coatings, their use, for practical purposes, is often limited to substrates that can withstand such high temperatures or that can be heated to an appropriate temperature long enough for cure to occur.
Despite the desirability of low-cure powder compositions, two problems have prevented their widespread production and use-their mechanical stability and their chemical stability. Conventional powders that use resins with a glass transition temperature (“Tg”) lower than 60° C. may encounter package stability problems, especially after prolonged heat exposure, and become fused, sintered or clumpy within days. Similarly, prolonged heat exposure can destroy the chemical stability of a powder if it includes crosslinkers that react at temperatures below about 170° C.; if a crosslinker with a lower cure temperature is used, cure may be initiated during storage even though the film has not been formed. The premature gelation that occurs in these powder formulations results in coatings having shortened gel times. It is not unusual for low-cure powders to lose >50 percent of their gel time as a result of the premature gelation.
Problems encountered when a powder loses either mechanical or chemical stability can be severe. Poor mechanical stability creates obvious handling, application and appearance issues. Poor chemical stability creates subtler yet just as problematic issues. For example, a powder that has poor chemical stability will fluidize and apply like virgin powder, but because it has advanced in reactivity (i.e. undergone some premature gelation), it demonstrates restricted flow or no flow at all during cure. The result can be a coating having an “orange peel” appearance, a rough texture or gel bodies.
Ideally, a powder should not lose its handling properties under elevated temperature storage and the gel time should remain the same as that of the virgin material. To achieve this, powders are typically formulated with resins having a Tg greater than about 60° C. and/or crosslinkers that react at temperatures of about 170° C. or greater. Such powders, however, are not low cure. Low-cure powders having lower Tg resins or lower temperature crosslinkers can require expensive storage under refrigeration and air-conditioned application facilities to overcome their inherent lack of stability, or must be prepared using special techniques.
Thus, there is a need in the coatings art for low-cure powder coatings having a wide range of application, which also have an acceptable level of durability when cured on the finished product and good stability at room temperature.